The need to provide environmentally correct and cost effective solutions for the refuse generated in the Unites States became apparent in the late 1960's. At that time, refuse disposal was mainly by land filling and to a lesser extent incineration. It was recognized that landfill space was a finite resource and that refuse itself was an inherent fuel resource that could be utilized to displace other more costly fuel sources in the generation of process steam and electricity.
A refuse to energy plant is composed of several separate and distinct areas. These include: (1) refuse receiving, handling and storage; (2) refuse combustion; (3) heat recovery and electrical generation; and (4) environmental control. The refuse combustion system is of particular interest herein and it typically is composed of the following: (1) refuse feed hopper and chute, (2) ram feeder, (3) grate combustion system, (4) lower furnace combustion zone, and (5) grate ash discharge system. Current conditions require that these systems operate over extended periods of time with limited maintenance. Initial operating results for these types of facilities exhibited extensive maintenance and downtime.
Refuse is introduced to the grate combustion system via a charging hopper and feed chute. Typically, a crane, or in some cases, a front end loader picks up a quantity of refuse from the site receiving and storage area and deposits it into the charging hopper. This charging hopper has a large plan area to facilitate this operation and acts as a funnel to feed the refuse to the feed chute. The feed chute typically is rectangular in cross section and has slightly divergent sides. The width of the feed chute approximates the width of the grate to facilitate uniform refuse fuel flow across the unit. The feed chute and the lower part of the charging hopper are always kept full of refuse to maintain a seal between the combustion zone within the furnace enclosure and ambient, i.e. the exterior of the furnace enclosure.
Refuse from the feed chute exits to a flat, table top surface, directly below. This surface provides a staging area for the refuse to move out onto the grate in a controlled manner beginning the combustion process. A ram feeder, which is a plow-type device, operates on top of this table and is hydraulically driven at a predetermined speed to push the refuse onto the grate. The feeder is considered a volumetric flow controller as it pushes a volume of refuse equal to its height by the plan area of the feed chute discharge opening. The feeding portion of the unit involves the group operation of several parallel ram feeders across the full width of the grate system to insure equal fuel loadings across the unit. Accordingly, refuse is pushed off the table and onto the grate system to start the combustion process within the furnace enclosure.
The speed of the ram in its forward sequence is controlled to provide refuse at a predetermined combustion rate. The ram always retracts at a relatively fast rate to avoid subjecting it to furnace radiant conditions at its most forward position when there is only a limited pile of refuse between the flow and the combustion zone.
The carriage system or drive end of the ram is a cart device riding on small steel wheels connected to the hydraulic ram. The wheels include a conventional shaft and greased bearing arrangement and ride on a flat rail.
The ability of the ram feeder to function properly requires that the wheel driven end operate uniformly in its forward and reverse stroke to maintain ram alignment. Experience has shown that over a several month period, potential areas of failure exist: (1) trash siftings interfere with the ability of the wheels to roll on the rails in a horizontal and uniform mode; (2) trash siftings would cause uneven wear of the wheels; and (3) failure of the greasing mechanism would cause binding and flattening of the round wheels. These occurrences would cause the wheel driven end to seize and move off the rails allowing the ram travel to be in a nonaligned stroke pattern interfering with adjacent rams and increases the required moving force. Accordingly, wheel-driven carriage systems are maintenance intensive and expensive. Further, left unchecked, this interference would cause the rams to eventually destroy each other when this side by side interference occurred during the travel cycle. Additional damage would occur on the riding or carriage table area side walls and support structure once the ram was damaged.